Question

I try to predict pKa values of acids (in water), computing their energy of deprotonation with quantum chemistry; and I found that the slope of the linear correlation pK vs Energy is more close to the theoretical value, if the computation uses an advanced method of computing the solvent effects. I try to explain this obsevation theoretically. Please comment, whether the following argumentation is correct:

Strong acids contain more polar OH fragments (in the neutral form), so strong acids in the neutral form are more strongly stabilized in polar solvents than in non-polar ones. As a result, in polar solvents, compared to non-polar solvents, strong acids become slightly weaker, and weak acids become slightly stronger.

Is this statement true? If we take a number of e.g. acids and measure their pKa in water and non-polar solvents, will it turn out that a polar solvent slightly reduces the dependence of pKa on the properties of the molecules themselves, i.e. in polar solvents, strong acids become a little weaker, and weak acids a little stronger due to the solvent?

In other words, I want to find a data on pK values of acids in non-polar solvents (or at least less polar than water), and check the correlation between the pKa values in water and in these solvents; the most important value is the slope of this correlation (whether it is not 1.0).

Answer 1

The statement is correct; polar solvents like water can have a stabilizing effect on the charged species of acids, leading to a reduced dependence of pKa on molecular properties and the occurrence of the leveling effect where strong acids appear equal in strength due to complete ionization.

Step-by-step explanation:

The observation that an advanced method computing solvent effects provides a linear correlation closer to the theoretical value can be explained by considering the properties of polar solvents like water, which can stabilize charged species.

This stabilization affects the ionization of acids and, thus, their apparent strength or pKa values. Strong acids with more polar OH groups are more stabilized in polar solvents compared to non-polar ones, which can result in an apparent decrease in acid strength.

Conversely, in polar solvents, weak acids can exhibit an apparent increase in strength due to better stabilization of their charged forms. Therefore, it is true that polar solvents like water can slightly reduce the dependence of pKa on the molecular properties themselves, highlighting the importance of solvent effects in acid strength determination.

This is coherent with the concept of the leveling effect, which describes the phenomenon where all strong acids in water are leveled to the strength of the hydronium ion, H3O+, since they fully dissociate and no stronger acids can exist in aqueous solution.

Measuring acid strength in non-polar or less polar solvents could indeed show a different slope of the correlation between pKa values compared to those in water, due to the absence of this leveling effect and the reduced stabilization of charged species.